A prospective cohort study has been conducted in Tasmania since 1988. The first finding from this study was that infants who slept prone (on tummy) at one month of age were at over four times increased risk of SIDS. Public health activities to reduce the prevalence of the prone infant sleeping position has coincided with a decline in the rate of SIDS in Tasmania. LONG-TERM OBJECTIVES: 1. To investigate the aetiology of SIDS using a prospective approach which will allow the collection of pertinent biological measurements and avoid the problem of recall bias. 2. To evaluate the independent contribution of changes in infant sleep position to the decline in SIDS rate. 3. To examine the effect of infant sleep position in relation to other health outcomes. 4. To assess the extent of recall bias present in the setting of bereaved parent interview. EXPERIMENTAL DESIGN AND METHODS: Source population - 94% of Tasmanian live births Study sample - 14OO infants (2O% of Tasmanian live births) who are at higher risk of SIDS using a perinatal scoring system annually Study measurements-sociodemographic, biomedical and environmental factors measured at three stages - 4 days, 1 month, 1O weeks of age Study outcome - SIDS is defined as the sudden unexpected death of an infant less than twelve months of age which is not explained by postmortem examination death scene investigation or clinical history. Study analysis - includes multiple logistic regression analysis of exposure-disease associations.